As long as 4 boxes are Linux it makes life easier.
I hope you know IP addresses of your asterisk.
I will use your picture:
Asteris 1 ------- FWA ---------- FWB -x--------- Asterisk 2
Please, on FWA execute command:
tcpdump -nnt src <IP of the Asterisk 1> -i <FWA INPUT interface>
You will see what asterisk #1 sends to FWA. Then you can execute it on FWB and compare those outputs.
It gives you an idea of what comes to FWA (without any changes by firewall) and what FWB receives.
Then execute that command on Asterisk 2 you will see what comes through FWB.
Try, it may give you many interesting details.
P.S. I added input interface, because FW has 2 interfaces, where tcpdump can catch source IP, so you need to specify one you need.
Last edited by nimnull22; 06-11-2010 at 03:59 PM.